Device and method for force phenotyping of cells for high-throughput screening and analysis

ABSTRACT

A system for assaying forces applied by cells includes an optically transparent substrate comprising a soft material having a Young&#39;s modulus within the range of about 3 kPa to about 100 kPa. An array of molecular patterns is disposed on a surface of the optically transparent substrate, the molecular patterns include fluorophore-conjugated patterns adherent to cells. The system includes at least one light source configured to excite the fluorophore-conjugated patterns and an imaging device configured to capture fluorescent light emitted from the fluorophore-conjugated patterns. Dimensional changes in the size of the patterns are used to determine contractile forces imparted by cells located on the patterns.

RELATED APPLICATION

This Application is a U.S. National Stage filing under 35 U.S.C. § 371of International Application No. PCT/US2015/023136, filed Mar. 27, 2015,which claims priority to U.S. Provisional Patent Application No.61/972,171 filed on Mar. 28, 2014, which is hereby incorporated byreference in its entirety. Priority is claimed pursuant to 35 U.S.C. §119.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under OD007113, awardedby the National Institutes of Health. The Government has certain rightsin the invention.

FIELD OF THE INVENTION

The technical field generally relates methods and devices used fordetermining and/or assaying forces applied by cells.

BACKGROUND

Current technologies used to study cell contractile forces includetraction force microscopy (TFM) of substrates having embeddedfluorescent particles. In TFM, cells are cultured as a monolayer on thesurface of a thin substrate with fluorescent microspheres (e.g., latexbeads) embedded therein. In another method, microfabricated siliconeelastomeric post arrays have been developed for measuring the tractionforces of adherent cells. See e.g., Sniadecki et al., MicrofabricatedSilicone Elastomeric Post Arrays for Measuring Traction Forces ofAdherent Cells, Methods in Cell Biology, Vol. 83 (2007). In this methoda vector map of traction forces is obtained by measuring the deflectionof each micropost. Cellular contractile forces can also be measuredindirectly by atomic force microscopy (AFM). AFM has been used tomeasure stiffness of cell cytoskeleton, which is broadly correlated tocontractility through myosin contraction of the actin cytoskeleton.These approaches have been used to map stem cells, and correlated to themetastatic potential of cancer cells. Elastomeric micropillars have beenused to track the contractility of stem cells, and researchers havefound variation in contractility during differentiation. See J. Fu, etal., Mechanical regulation of cell function with geometrically modulatedelastomeric substrates, Nature Methods, Vol. 7, pp. 733-739, (2010).

TFM, which operates by mapping forces on the substrate as cellstranslocate across soft substrates and stretch the substrate, has beenused to map forces in the leading vs. retracting edge during cellmigration. Despite the usefulness of these methodologies, thesetechniques are not amenable to simple, high-throughput extraction ofcontractility measures from mixed populations of cells with raresub-populations that need high numbers to statistically sample. Thesetypes of samples would be seen from a patient sample or mixed culture.Additionally, these previous techniques require high-resolution imagingand precise focusing of optical systems which make them less compatiblefor high-throughput analysis needed for example in attempting to screenfor effect of a large number of drugs on force production. There thus isa need for better methods and systems for identifying and quantifyingthe forces applied by cells, including populations of cells with rarephenotypes.

SUMMARY

In one aspect of the invention, a system for assaying forces applied bycells includes an optically transparent substrate comprising a softmaterial having a Young's modulus within the range of about 3 kPa toabout 100 kPa. An array of molecular patterns is disposed on a surfaceof the optically transparent substrate, the molecular pattern comprisingfluorophore-conjugated patterns adherent to cells. The system includesat least one light source configured to excite thefluorophore-conjugated patterns. For example a first light source may bean excitation light source for the fluorophore-conjugated patterns. Thesecond light source may be used to excite or pump another fluorophore.For example, another different fluorophore may be used to detect anorganelle within the cell (e.g., cell nucleus). The light sources mayinclude light emitting diodes (LEDs), laser diodes, or the like. Thesystem includes an imaging device that is configured to capturefluorescent light emitted from the fluorophore-conjugated patterns. Theimaging device may also be used to capture fluorescent light emittedfrom fluorescently labelled organelles such as the cell nuclei. The cellnuclei can be used to identify those fluorophore-conjugated patterns inwhich no cells adhere, one (1) cell adheres, or two (2) or more cellsadhere. According to some embodiments, the fluorophore-conjugatedpatterns where no cells adhere are used as a control. In addition, insome aspects of the invention, the fluorophore-conjugated patterns wheretwo or more cells adhere are discarded. However, in some embodiments,useful information may be obtained from fluorophore-conjugated patternswhere two or more cells have adhered. Furthermore, immunofluorescence orother techniques may be used to introduce yet anotherfluorophore-conjugated molecule emitting fluorescent light at a thirdwavelength for the purpose of labeling and identifying, for example,surface markers on the cells and relating those to the informationobtained from the imaged fluorophore-conjugated patterns.

In another embodiment of the invention, a system for assaying forcesapplied by cells includes an optically transparent substrate comprisinga soft material having a Young's modulus within the range of about 3 kPato about 100 kPa. The system includes an array of molecular patternsdisposed on a surface of the optically transparent substrate, themolecular pattern comprising fluorophore-conjugated patterns adherent tocells. A second substrate containing a plurality of apertures therein issecured to the surface of the optically transparent substrate to form aplurality of wells wherein each well contains one or more molecularpatterns therein (e.g., a subset of patterns can be contained withineach well). For example, the second substrate could include 96 wells asis used in a conventional 96 well test plate (of course other number ofwells could be used). The system includes at least one light sourceconfigured to excite the fluorophore-conjugated patterns. The excitedfluorophore-conjugated patterns emit fluorescent light that is capturedby an imaging device. A computing device is configured to receive imagesfrom the imaging device and measures a dimensional change of thefluorophore-conjugated patterns having cells disposed thereon. Thedimensional change may include a contraction of the pattern (i.e.,pattern gets smaller in some respect) or it may include a relaxation(i.e., pattern gets larger in some respect), or a skewing (i.e., patterngets smaller or larger in a non-uniform manner).

In another aspect of the invention, a method of using the systemdescribed above includes the operations of loading the plurality ofwells with cells so that at least some of the cells adhere to thefluorophore-conjugated patterns. In one embodiment, the array of wellscan be used to analyze compounds or drugs for their ability to affectchanges in protein targets, signaling pathways, the cellular membrane orcytoskeletal structures that impart force on the underlying patternedsubstrate. For example, some of the wells or each well is loaded with adifferent compound or drug and the plurality of wells are illuminatedwith the at least one light source simultaneously or in a well-by-wellsequence in time. The fluorophore-conjugated patterns from the pluralityof wells are imaged with the imaging device and a dimensional change ofthe fluorophore-conjugated patterns within the wells is measured.Certain cells or sub-populations of cells can be identified by adimensional change that exhibits a certain characteristic. For example,the computer may identify those cells having a dimensional change aboveor below a threshold value or a dimensional change within a specificrange.

In another aspect of the invention, a method of identifying the forcephenotype of cells includes providing an optically transparent substratecomprising a soft material having a Young's modulus within the range ofabout 3 kPa to about 100 kPa. The optically transparent substrate has anarray of molecular patterns disposed on a surface thereof, the molecularpattern comprising fluorophore-conjugated patterns adherent to cells.Cells are loaded onto the optically transparent substrate, wherein atleast some of the molecular patterns have one or more cells adheredthereto. The fluorophore-conjugated patterns are illuminated and imagesare captured of fluorescent light emitted from thefluorophore-conjugated patterns with an imaging device. A dimensionalchange of the fluorophore-conjugated patterns is measured with acomputing device that receives the images. The cells are thencategorized or classified based at least in part on the measureddimensional change.

In another aspect of the invention, a method of forming a substratehaving an array of fluorescent-conjugated molecular patterns includesproviding an optically transparent substrate. A layer ofpolydimethylsiloxane (PDMS) is formed on the optically transparentsubstrate having a Young's modulus within the range of about 3 kPa toabout 100 kPa. Next, a photoresist is patterned on the layer of PDMS.Fluorescently-conjugated molecules are attached to the layer of PDMSpatterned with the photoresist. The photoresist is then removed.

In another aspect of the invention, a method of forming a substratehaving an array of fluorescent-conjugated molecular patterns includespreparing a polydimethylsiloxane (PDMS)-based stamp having a desiredarray of patterns. A plurality of fluorescently-conjugated molecules isattached to the array of patterns of the stamp. The stamp is pressed totransfer the molecular pattern onto a layer of dextran spun to achieve athin layer on a flat substrate. A soft layer of PDMS is formed on thestamped dextran, where the PDMS layer has a Young's modulus within therange of about 3 kPa to about 100 kPa. The dextran layer is thensacrificed to release the soft layer of PDMS. The soft layer of PDMS isthen mounted on an optically transparent substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates one embodiment of a system for assayingand measuring forces applied by cells.

FIG. 2A illustrates illustrative shapes and configurations forfluorescent molecular patterns that are used as part of the system.

FIG. 2B illustrates an example of a unit cell containing multiplefluorescent molecular patterns arranged to increase cell density whileminimizing cell-to-cell coupling.

FIG. 3 illustrates a perspective view of an optically transparentsubstrate with a layer of soft polymer material disposed on thesubstrate and containing a plurality of fluorescent molecular patternsthereon. Also illustrated is a top view as well as a magnified view of asingle pattern.

FIG. 4A illustrates a fluorescent image taken of an array of molecularpatterns disposed on a layer. Successively magnified images areillustrated with the final image showing two (2) such patterns. Onepattern (upper) has no cells adhered thereto. The other pattern (lower)has contracted inward in response to forces exerted on the layer by thecell.

FIG. 4B illustrates a brightfield image of a portion of the layercontaining the fluorescent pattern. Cells are seen on some of themolecular patterns.

FIG. 5 illustrates still images from a time lapse video of a vascularsmooth muscle cell contracting a green fluorescent fibrinogen andnon-fluorescent fibronectin pattern. As time progresses, one can see nowthe cross-shaped pattern compresses in both directions (perpendicularaxes) but does not peel from the substrate.

FIG. 6A schematically illustrates another embodiment of a system forassaying and measuring forces applied by cells. This embodiment uses aplurality of wells. A side view of the well-based device is illustrated.

FIG. 6B illustrates a top view of the device of FIG. 6A. 96 wells areillustrated although more or less can be used (e.g., 384).

FIG. 7 illustrates successively magnified images of an embodiment of thetype illustrated in FIGS. 6A and 6B.

FIG. 8 illustrates how representative input images of the layer that isobtained with the imaging device are processed by the imaging processingsoftware to generate deflection data.

FIG. 9 illustrates the operations or steps of two different methods usedto create the soft polymer layer atop the optically transparent layerhaving molecular patterns formed thereon.

FIG. 10 illustrates a single pattern showing the identified center ofthe pattern along with the four (4) measurements for each “arm” of thepattern.

FIG. 11A illustrates the contraction of airway smooth muscle (ASM) cellsthat were exposed to acetylcholine, enodthelin-1, and histamine. ML-7was also added after contraction and was shown to relax smooth musclecontraction. This is seen by the reduction in deflection amount that wasmeasured after addition of myosin light chain kinase inhibitor ML-7(+ML-7). The negative control is also illustrated (−ML-7). Deflectionson the y-axis are in micrometers.

FIG. 11B illustrates the contraction of ASM cells in response histamineas well as the resulting relation in response to the addition ofalbuterol (+ Albuterol).

FIG. 12A illustrates respective deflection graphs as function ofincreasing blebbistatin concentration for HeLa cells.

FIG. 12B illustrates a dose-response curve for blebbistatinconcentration as a function of percentage relaxation for HeLa cells.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1 schematically illustrates one embodiment of a system 10 forassaying forces applied by cells 11. In this embodiment, a layer 12 ofsoft polymer material is disposed atop an optically transparent, rigidsubstrate 14. The layer 12 of soft polymer material includes one or moremolecular patterns 16 as described in more detail below. The molecularpatterns 16 are formed within or applied directly onto the layer 12 ofsoft polymer material and are fluorophore-conjugated patterns 16 thatare adherent to cells 11 that are placed onto the layer 12 of softpolymer material. The fluorophore-conjugated molecular patterns 16 areused to precisely control the locations and shapes of adhered cells 11,and for direct visualization of the forces the adhered cells 11 apply(or, conversely, don't apply) to the layer 12.

The layer 12 is soft enough such that the soft polymer material thatforms the layer 12 can be deformed at least 1 μm by forces on the orderof magnitude of those applied by cells 11, i.e., forces within the rangeof pN-nN. The height of this layer 12 of soft polymer material may varybut is generally less then around 20 μm in height. In one embodiment,the layer 12 of soft polymer material includes polydimethylsiloxane(PDMS). More particularly, the layer 12 of PDMS has a Young's moduluswithin the range of about 3 kPa to about 100 kPa and more preferablywithin the range of about 3 kPa to about 20 kPa. The softness of thePDMS layer 12 may be tuned by using a high base to crosslinker ratio(e.g., within a range of around 70:1 to around 20:1).

As seen in FIG. 1, the layer 12 of soft polymer material is secured toone surface of an optically transparent substrate 14. The opticallytransparent substrate 14 may include a glass material (e.g., glassslide, coverslip, or the like). In one aspect of the invention, lowelasticity PDMS is spun onto the optically transparent substrate 14 toproduce the layer 12. The adhesion between the PDMS layer 12 and theoptically transparent substrate can be enhanced through modifications tothe surface of the optically transparent substrate 14 prior to spinning.For example, for an optically transparent substrate 14 formed fromglass, plasma treatment followed by silanization prior to spin coatingcan be used. Alternatively, as explained below with respect to theprocess of FIG. 9, the PDMS layer 12 may be spun onto a dextran-coatedflat substrate if the pattern transfer method is employed for creatingthe molecular patterns.

Still referring to FIG. 1, the PDMS layer 12 includes a plurality offluorescently-conjugated molecular patterns 16. In one aspect, thefluorescently-conjugated molecular patterns 16 are formed as part of anarray. The fluorescently-conjugated molecular patterns 16 are applieddirectly onto the surface of the PDMS layer 12. In one embodiment, themolecular pattern 16 may be a single labeled protein such as humanIgG-FITC or it may be a combination or mixture of any number ofproteins, of which one must serve as a reporter protein and befluorescently labeled while the others engage the cells 11 and promotespecific behaviors (e.g., fibrinogen-FITC as a reporter and unlabeledfibronectin to promote adhesion). The molecular pattern 16 can be of anyshape and can either be symmetric or asymmetric. Preferably, molecularpattern 16 has a minimal line-width of about 3 microns. The molecularpattern 16 may have a number of shapes or configurations including, butnot limited to, symmetric and asymmetric crosses, squares, circles(filled or un-filled), ellipses, ovals, rings, teardrop-like shapes, andothers. FIG. 2A illustrates several different shapes that can be usedfor the molecular patterns 16. Shapes that are asymmetric can provideinformation on asymmetric forces that cells 11 apply. The variouspattern shapes can be manipulated (e.g., sheared, enlarged, etc.) toallow for additional features of cell contractility to be measured.Since the molecular patterns 16 are within an order of magnitude of thesize of cells 11, over 100,000 molecular patterns 16 can fit on a singlesubstrate the size of a glass coverslip.

FIG. 2B illustrates one configuration of molecular patterns 16 that arearranged in a unit cell 18 configuration to improve the informationdensity of the molecular patterns 16. If portions of molecular patterns16 are too close to one another, mechanical coupling of adjacent cells11 can occur which will modify the apparent stiffness and canpotentially lead to oscillations in contractions. The unit cell 18configuration of FIG. 2B places regions of high traction away from eachother such that higher densities of cells 11 can be achieved withoutcross-talk. Note that the unit cell 18 configuration may be repeated.

Referring back to FIG. 1, the system 10 further includes a first lightsource 20 which may include wavelength filtered arc lamp, a lightemitting diode (LED) or laser diode or multiple LEDs or laser diodes.The first light source 20 emits light at a wavelength or wavelengthrange that excites the fluorophore that is part of thefluorophore-conjugated molecular patterns 16. In one optional aspect,the first light source 20 may be a tunable light source 20 in whichdifferent wavelengths can be selected. In such an embodiment, a singlelight source may be used instead of multiple light sources. In onepreferred embodiment of the invention, the system 10 includes a secondlight source 22 that emits light at a different wavelength from thefirst light source 20. The second light source 22 may be used to exciteanother different fluorophore than the fluorophore-conjugated molecularpatterns 16. In one aspect, the first and second light sources 20, 22may be integrated into another device such as a fluorescent microscope28. For example, as explained herein, the nucleus of the cell 11 may bestained with a fluorescent dye or other fluorescent reporter that emitslight in response to excitation by the second light source 22. The lightthat is emitted from the nucleus of the cell 11 can be used to identifyand count the number of cells 11 that have adhered to the molecularpatterns 16. For example, imaging of the array of molecular patterns 16may indicate the presence of two (2) cell nuclei disposed above a singlemolecular pattern 16 which is indicative that there are two cells 11located on the single molecular pattern 16. Likewise, the absence of anystained nuclei above a molecular pattern 16 reflects that zero (0) cells11 are located on that particular location of the molecular pattern 16.Additional wavelength excitation sources and emission filters may beused to characterize other molecular properties of adhered cells (e.g.,immunofluorescence of cell surface proteins indicative of a specificcell type or state with fluorescently labeled antibodies) in combinationwith applied force and nuclear presence.

Stained cell nuclei may be counted by using two gates: (1) size, and (2)solidity. Larger sized objects must have high solidity to be considereda single nucleus. Smaller objects can have lower solidity (subject to aminimum threshold) and still be considered as having a single nucleuspresent. If the conditions are not satisfied, the object is rejected ashaving 2+ nuclei. The MATLAB software program and the functions “im2bw,”“bwareaopen,” “bwlabel,” and “regionprops” can be used to determine thesize and solidity of objects as well as perform the necessary binaryimage operations. Initially, the image containing DAPI (or other nuclearstain) is turned into a binary image and small objects are removed.Next, the number of distinct objects is determined in the binary image.If there are 0 or 2+ objects, these are classified as such and the nextpattern location is chosen. If there is what appears to be a singleobject, the area (A) and the solidity (S) are calculated. The Solidity(S) is defined as the fraction of area of the object to the area of thesmallest convex polygon (P) encompassing the object (S=A/P). Theappropriate threshold for the area (A) is established based on the celltype. Smaller objects are likely to be a single nucleus and will have alower threshold for an acceptable S value while larger objects are morelikely to be two or more overlapping nuclei and therefore will have ahigher minimum S value to be deemed a single nucleus. After establishingthe threshold cutoff for the S value, the measured S value is comparedto the threshold value. If the measured S value is higher than thethreshold or cutoff value then the object is considered a singlenucleus. If the measured S value is below the threshold value it isconsidered 2+ nuclei.

Still referring to FIG. 1, an optional filter 24 is provided in theoptical path that formed between the layer 12 and an imaging device 26.The optional filter 24 is used to filter out excitation light from thelight source(s) 20, 22 while permitting the passage of fluorescent lightthat is emitted from the fluorescent dye or probes. The light sources20, 22 may also be oriented obliquely with respect to the layer 12 whichcan assist with rejection/filtering of the excitation light. Theoptional filter 24 may be secured to the optically transparent substrate14 or it may even be removable within the optical path as opposed tobeing located within the imaging device 26. Still referring to FIG. 1,the system 10 includes an imaging device 26 that is positioned along theoptical path so as to acquire images of the fluorophore-conjugatedmolecular patterns 16 including optional images of stained features ororganelles within the cells 11. The imaging device 26 may includeoptical components of an inverted fluorescent microscope 28. The opticalcomponents include, for example lenses 25 that are used to magnify theimage of the layer 12 containing the fluorescent molecular patterns 16.Typical magnification used for the fluorescent magnification is 10×which allows more rapid imaging of a large number of cells to obtainstatistically relevant data in a shorter time than in previoustechniques to measure cell forces. The fluorescent microscope 28includes a CCD or CMOS imaging sensor 30 that is used to capture twodimensional digital image frames. The imaging device 26 is able tocapture two dimensional image frames which contain fluorescent images ofthe fluorophore-conjugated molecular patterns 16. The fluorescentmicroscope 28 may include filters 24 therein. For example, cells 11 canbe imaged with UV and blue excitation channels corresponding with blueand green emission filters, respectively. The fluorescent microscope 28may include “green” (e.g., 488 nm) or “red” (e.g., 532 nm) channels thatare used for imaging the fluorescent pattern 16. A “blue” channel (e.g.,about 360 nm) may be used to image DAPI or Hoechst stains that are usedto label cell nuclei.

As noted herein, the fluorophore-conjugated molecular patterns 16 may bepatterned in an array on the layer 12. In particular, the location ofeach fluorophore-conjugated molecular pattern 16 can be identified sothat contractile motion of any particular molecular pattern 16 can beassociated with a particular cell 11 or cells 11 that may be adhered tothat spot. In this regard, one or more landmarks may be provided on thelayer 12 or optically transparent substrate 14 so that specificlocations can be mapped. In one example, the molecular patterns 16 maybe arrayed in rows and columns. For example, unit cells 18 may bearrayed in rows and columns in the layer 12. In one example that isdescribed below in more detail, the molecular patterns 16 are containedwithin individual wells or chambers so that separate reaction areas areprovided. These segregated wells or chambers can be used to testcompounds or pharmaceutical compositions on cells 11 to investigatetheir ability to affect contractile movements or force generation, forexample.

Still referring to FIG. 1, the system 10 includes a computing device 32that is operably coupled to the imaging device 26. The computing device32 may include a computer which may be a personal computer, laptop,tablet, or the like. The computing device 32 is associated with adisplay 34 that can be used to display images of the molecular patterns16 in the layer 12 along with other fluorescent images of organelles orthe like that are stained or otherwise emit fluorescent light. Data thatis generated by the imaging processing software 36 executed by thecomputing device 32 may also be presented on the display 34. Data caninclude, for example, dimensional changes of the molecular patterns 16,rates of change of the dimensional change in the molecular patterns 16.Data can also include histogram or binned data pertaining to cells 11that are adhered to the molecular patterns 16. For example, cells 11that have dimensional changes exceeding a particular threshold amount orrange can be identified. The computing device 32 includes at least oneprocessor 38 that is used to execute image processing software 36 asdescribed below.

A description will be given of how the image processing software 36processes the digital image frames obtained by the imaging device 26.Image frames may be saved with the .TIFF formal to preserve informationfor analysis or later viewing. The image frames may also optionally besaved in the compressed .PNG format. Of course, any valid image format(e.g., .jpg, .bmp, or the like can be used). After cells have beenplaced or otherwise incubated with the layer 12 and allowed to adhere tothe molecular patterns 16, the array of patterns 16 is then imaged withthe imaging device 26. The imaging device 26 may obtain fluorescentimages of the molecular patterns 16 before, during, or after the cells11 have undergone contractile movement or, conversely, relaxation. Thedynamic range of the pixel intensities may be adjusted post-imagingusing, for example, ImageJ so that the patterns 16 can be visualized.

The image processing software 36 identifies all instances of a givenmolecular pattern 16 in an image stack and determines how many cells(e.g., 0, 1, or 2+) are adhered to each pattern 16 based on images ofthose cells 11 with stained nuclei. A MATLAB script opens the imagefiles and pairs the stained nuclei images (e.g., DAPI images) with thefluorescent images of the patterns 16 (e.g., FITC images). The imageprocessing software 36 calculates characteristic pixel length(s) of agiven pattern 16 for the cases of no cells adhered and one (1) celladhered. The ‘FITC’ images may be linearly scaled up with, for example,the MATLAB function ‘imresize’ to improve resolution of thesemeasurements. For those patterns with zero cells adhered thereto, thecenter of each pattern 16 may be determined using binary center-of-masscalculations. For those patterns 16 with one (1) cell adhered thereto, aseries of binary dilation and erosion operations, followed by a binarycenter-of-mass calculation on patterns is performed. Theerosion/dilation procedure is used to remove the potential asymmetry inthe pattern 16 arising from cell-induced deformations that wouldotherwise impact the location of the center of mass. Patterns 16 withtwo or more nuclei are not analyzed. For each processed pattern 16(those that are crosses or “X” shaped), the diagonal distance in pixelsis measured between the calculated center and the bright edge of thepattern 16 in each direction as illustrated in FIG. 10. This results infour (4) measurements per pattern 16. These measurements are stored,converted to microns based on the magnification used (typically 10×).These lengths are normalized to the median of those measured in patterns16 with no cells (nominally 0 pixel deformations which is used as acontrol) and a relative distribution plot for patterns with one (1) cellis generated (see FIG. 8). Cropped images of each pattern 16 screened bythe image processing software 36 in a given location and the images ofthe corresponding cell nuclei on those patterns 16 are saved and may bereviewed if an execution error is suspected. FIG. 8 illustrates examplesof cropped DAPI and FITC images.

If a circular shaped pattern 16 is used, the MATLAB “imfindcircles”function can be used to locate circles falling within a certain radiusrange, and the center (x,y) and the radius (r, in pixels) for eachcircle can be stored. The same nuclei counting function as describedabove is used to determine the number of cells 11 on each pattern 16.The radii in pixels are converted to microns, and the differencesbetween each of the radii of circle-patterns with 1 cell and the medianof the radii of circle-patterns with 0 cells can then be plotted as adistribution as described above with respect to the non-circular pattern16.

The image processing software 36 can take absolute measurements ofpatterns 16 if the dynamic analysis is used. Thus, the imagingprocessing software 36 is able to calculate dimensional changes of themolecular patterns 16 occurring in response to cytoskeletal changes inthe cells 11. The dimensional changes can be monitored in real time fordynamic monitoring of cell-substrate interaction or dimensional changescan be measured using end-point analysis where forces applied on thelayer 12 reach a steady-state.

FIG. 3 illustrates a perspective view of the layer 12 of soft polymerdisposed atop an optically transparent substrate 14 along with a topview of the layer 12. A magnified view of a single pattern 16 containinga single cell 11 is also illustrated in FIG. 3. Different molecularpatterns 16 are illustrated formed in an array on the layer 12. Cells 11are illustrated as being adhered to some of the fluorescently adhesivemolecular patterns 16. FIG. 3 also illustrates a magnified view of asingle cell 11 (with nucleus) adhered to one of the molecular patterns16. In this example, the molecular pattern 16 is in the shape of an “X”that has contracted in response to the contracting cell 11. The originalshape of the “X” pattern 16 is illustrated in outline α as well as thecontracted shape β.

FIG. 4A illustrates a fluorescent image taken of an array of molecularpatterns 16 disposed on a layer 12. Successively magnified images areillustrated with the final image showing two (2) such patterns. Onepattern 16 (upper) has no cells 11 adhered thereto and so the pattern 16retains its original shape. The other pattern 16 (lower) has contractedinward in response to forces exerted on the layer 12 by the cell 11.FIG. 4B illustrates a brightfield image of a portion of the layer 12containing the fluorescent pattern 16. Cells 11 are seen on some of themolecular patterns 12. FIG. 5 illustrates a time lapse video stillimages of vascular smooth muscle cell contracting a green fluorescentfibronectin pattern 16. As time progresses, one can see now thecross-shaped pattern 16 compresses in both directions (perpendicularaxes).

FIGS. 6A and 6B illustrate another embodiment of the invention. In thisembodiment, a substrate 40 such as a plate having apertures 42 formedtherein (non-tapered) is adhered to the layer 12 such that individualwells 44 are formed over the surface of the layer 12 with each well 44containing a plurality of fluorescent patterns 16. For example, eachwell 44 may circumscribe a region of the layer 12 such that largenumbers of fluorescent patterns 16 are formed therein (e.g., between10,000 and 100,000). Advantageously, each well 44 contains the samenumber of patterns 16 with the same pattern shapes. In this regard, eachwell 44 contains a similar reaction environment. This embodiment may beuseful for drug discovery applications. For example, a differentcompound or drug of interest or cocktail thereof can be loaded intodifferent wells 44 which can then be individually examined for therespective force response(s) of the adhered cells 11. Not only is thisembodiment useful for screening but it can also be used with differentconcentrations of the same compound or drug of interest to determinedose-response effects on contractility or cell viability.

The substrate 40 is adhered to the layer 12 and surface tension from thewetting solution ensures a quick and effective bond between thesubstrate and the layer 12. A roller applicator or the like may be usedto apply uniform pressure to the substrate 40 and/or opticallytransparent substrate 14 holding the layer 12. This results in awater-tight seal. In an alternative embodiment, an adhesive or clamp(not shown) may be used to secure the substrate 40 to the layer 12. Inthis embodiment, the wells 44 are filled with solution to keep the layer12 wet. Once cells 11 are ready to be seeded onto the patterns 16 withinthe wells 44, the wells 44 can be filled with media and incubated untilthe cell suspension is prepared and loaded into the wells 44.

FIG. 7 illustrates successively magnified images of an embodiment of thetype illustrated in FIG. 6. A 96 well substrate or plate 40 isillustrated. Six of the wells 44 are illustrated in a magnified viewfollowed by another magnified portion of the layer 12 within a singlewell 44. Finally, two different fluorescent patterns 16 are magnifiedand illustrated. The upper fluorescent pattern 16 contains no cells andillustrates a non-contracted pattern 16. Conversely, the lowerfluorescent pattern 16 illustrates significant contraction of the armsof the “X” shaped pattern 16.

Referring now to FIG. 8 a description of how the image processingsoftware 36 processes the images will now be described. FIG. 8illustrates how representative input images of the layer 12 that isobtained with the imaging device 26 are processed by the imagingprocessing software 36 to generate deflection data. Specifically, FIG. 8shows an image of the fluorescent patterns 16 and also illustrates animage of the same field of view with DAPI(4′,6-diamidino-2-phenylindole) stain. DAPI is a fluorescent stain thatbinds strongly to A-T rich regions in DNA. Each of these patterns 16 isidentified and measured automatically with the image processing software36. The image processing software 36 detects the center or centroid ofeach pattern 16 and denotes the location thereof (e.g., pixel locationin image frame). The image processing software 36 also finds theterminal ends of the same pattern 16 which in this case includes an “X”shape and stores the respective locations (again pixel locations). Theimage processing software 36 identifies those “X” shapes within thepattern 16 where no cells 11 have adhered. This is done by using theDAPI channel of the fluorescent microscope 28 to image fluorescentlylabeled cell nuclei and obtain the DAPI image. Alternatively, a livecell cytoplasmic stain such as calcein AM or Celltracker™ (LifeTechnologies, Inc.) series of dyes can be used as long as thefluorescent emission does not significantly overlap with the adhesivepattern fluorescent emission. FIG. 8 illustrates a corresponding DAPIimage of the same field of view of FIG. 8. The presence of the “dots” inthe DAPI image indicates the presence of a cell 11 on the pattern 16 dueto the stained nucleus. The image of the pattern 16 may have a number ofdata points associated with each pattern 16. For example, in the case ofcrosses or “X” shapes, a change may be defined by the change in distancefrom the cross or “X” center to the furthest points of each arm,totaling four (4) data points per pattern 16.

The image processing software 36 cross-references the image of thefluorescent pattern 16 with that obtained of the same field of view withthe DAPI image and identifies those patterns 16 that do not have anycells 11 adhered thereto by identifying the patterns 16 that do not haveany fluorescent nuclei. These patterns 16 are then used as the controls.For example, similarly shaped patterns with no cells 11 thereon may havetheir respective dimensions measured and a baseline established that isrepresentative of no deflection in the pattern 16. The imagingprocessing software 36 uses the DAPI channel (or other non-overlappingchannels for other cytoplasmic dyes) to identify those patterns 16 withonly a single cell 11 or multiple cells 11. In one preferred aspect ofthe invention, the patterns 16 with only a single cell 11 are used tomeasure contraction or relaxation of the layer 12 in response tocorresponding cellular force application changes. These images arecropped and saved for each identified pattern 16 and stored in adirectory for access in case of an execution error is suspected as wellas for confirming potential sub-populations and/or outlier data. Imageswith two or more cells 11 are discarded but in some embodiments, theseimages may be used. Changes in the dimension of the patterns 16 holdinga single cell 11 is calculated by measuring the terminal ends of eachthese patterns 16 and subtracting a mean or average made of these samepatterns 16 with no cells adhered thereto. In some embodiments, eachpattern 16 may be associated with multiple data points (e.g., crosses orX shapes may have four (4) data points). Alternatively, different datapoints from a single shape may be combined into an average or meandimensional change.

FIG. 8 illustrates respective DAPI and FITC images of single pattern 16with no cells adhered thereto. FIG. 8 also shows respective DAPI andFITC images of the same pattern 16 with a single cell adhered thereto.As seen in the FITC image of the pattern 16 with one cell 11, theterminal ends of the pattern 16 have contracted inward due to thecontractile forces placed on the layer 12 by the adhered cell 11. FIG. 8illustrates the measured deflection in microns of both the controlpattern locations (with no cell) and pattern locations with only asingle cell. All gathered measurements are normalized to the median ofthe control data which is set to zero. The control locations arecentered around zero (0) deflections with about +/− microns indeflection. Contrast this with the much wider deflection rangedemonstrated by those pattern locations that have a single cell.

The image processing software 36 can be used to measure dimensionalchanges along multiple different axes depending on the nature of themolecular pattern 16. For example, the pattern 16 may have a major and aminor axis that may be orthogonal to one another. In another embodiment,the pattern 16 may be circular shaped and the image processing software36 may be used to fit the pattern 16 with a circle that can be used tomeasure the diameter or radius of portions of the pattern 16. In someinstances, only a single dimension or axis is measured. In otherembodiments, multiple axes are measured to determine contractility.Contractility in one dimension may be different from contractility inanother dimension.

The image processing software 36 may run for end-point analysis offorces applied by cells 11 in steady-state and for dynamic monitoring ofcell-substrate interaction. For end-point analysis, cells 11 of interestare seeded and cultured on the layer 12 for the needed period of time(˜6 hrs. for cells that interact with the patterns solely through focaladhesions or <1 hr. for phagocytic cells). To end the experiment, thecells 11 may be fixed with 4% paraformaldehyde, stained with DAPI, andthe substrate is mounted onto a glass slide for ease of handling, andimaged at any time. For dynamic analysis, the layer 12 is securelymounted to the imaging device 26. After the desired field-of-view isselected, cells 11 are seeded on the layer 12 and time-lapse imaging isused to record the dynamics of the cell-substrate interaction. Thismodality is well suited for measuring individual cellular responses toexposures to drugs or gene modifications, contractility dynamics andforce profiles for cardiac myocytes or other beating cells, and forother measurements of rates of force application where it may be usefulin uncovering dynamics that are masked in end-point analysis data.Certain cell types, such as some phagocytic cells, however, are imaged“live” even for end-point analysis as it has been found thatparaformaldehyde treatment induces relaxation in certain cell types.

As an alternative to the imaging device 26 described above, imaging andanalysis may also be done using a wide-field smart-phone or tablet PCbased fluorescence and/or dark-field based microscopic imaging devices.See e.g., Wei, Q. et al. Fluorescent Imaging of Single Nanoparticles andViruses on a Smart Phone. ACS Nano 7, 9147-9155 (2013) or H. Zhu, etal., “Cost-effective and Compact Wide-field Fluorescent Imaging on aCell-phone”, Lab on a Chip (2010) to enable use of the invention in thefield, in resource poor settings, or simply at a lower cost. Fluorophoreexcitation can be achieved using light-emitting-diodes and/or laserdiodes that are placed at an oblique angle with respect to the opticalaxis of the imaging design, which can assist with therejection/filtering of the excitation beam. In this alternativeembodiment, the fluorescent microscope 28 is omitted and the camerafunctionality of the mobile electronic device is used to capture thefluorescent images of the patterns 16.

FIG. 9 illustrates two different methods that can be utilized to formdevice structure that has the optically transparent substrate 14, thelayer 12 of soft material, and the molecular patterns 16. These includean adsorption/lift-off method and a pattern transfer and incorporationmethod. The adsorption/lift-off method relies on non-specific adsorptionof adhesive molecules used as part of the pattern 16 as a mechanism ofmolecule-to-substrate attachment. First, in operation 100 an opticallytransparent substrate 14 such as glass is provided and treated withoxygen plasma and exposed to allyltrimethoxysilane in vacuum for 12hours. Next, as seen in operation 105, a thin film (e.g., less thanabout 20 microns in height) of soft PDMS having a Young's modulus uponcuring within the range of about 3 kPa to about 100 kPa is spun on theoptically transparent substrate. The softness is adjusted by controllingthe ratio of base-crosslinker which should be in the range of about 50:1to about 70:1 although numbers outside may work as well. In operation110, a thin layer (<1 μm height) of positive photoresist 50 is spun(e.g., 30 seconds at 2400 RPM) onto the soft PDMS layer 12 and isexposed to UV light through a photomask, designed with software such as“L-Edit,” with clear regions in the desired adhesive patterns 16. Thephotoresist 50 is developed and baked to create adhesive regions 16 onthe PDMS layer 12. The selected adhesive molecules (e.g.,fluorophore-conjugated molecules) are then allowed to non-specificallyadsorb to the PDMS layer 12 as seen in operation 115. Next, as seen inoperation 120, a lift-off process is performed using ˜1 min ofsonication and manual agitation in KOH developer to remove the remainingphotoresist 50. The ultra-soft PDMS layer 12 is now formed as seen inoperation 125 of FIG. 9 that includes the molecular patterns 16 formedthereon. The molecular patterns 16 preferably have a minimum line widthof around 3 μm. As one example, molecular patterns 16 may be formed withfibrinogen-Alexa Fluor® 488 and human fibronectin arranged in crosses(50 μm diagonal, 10 μm line-width) spaced 25 μm apart.

In operation 130, the structure that includes the optically transparentsubstrate 14 and the PDMS layer 12 is treated with a pluronic solution(e.g., Pluronic F-127<1% wt/vol) for 40 minutes to prevent non-specificadsorption of cells 11 off of the adhesive patterns 16 which wouldcomplicate data analysis following imaging. In operation 135, cells 11are seeded onto the PDMS layer 12 containing the molecular patterns 16.

FIG. 9 also illustrates the pattern transfer and incorporation methodthat is used to prepare an optically transparent substrate 14 with anultra-soft polymer layer 12 that has adhesive patterns 16 formedthereon. This method uses a pattern transfer and incorporation process.This method creates covalent bonds between the chosen adhesivemolecule(s) and the soft PDMS layer 12. First, as seen in operation 150soft lithography is used to create a stiff PDMS stamp 41 (10:1 base tocrosslinker ratio used to form the stamp 41) containing the desiredgeometrically shaped and sized patterns.

To form the stamp 41 a photoresist master mold is fabricated, usingsoftware such as “L-Edit” to design a metal photomask with the desiredpatterns. Fabrication of the chrome photomask is outsourced. A positivephotoresist such as SPR-220-7 (thick resist) is spin-coated onto asilicon wafer, soft baked, exposed through the metal (e.g., chrome)photomask, and developed until the master mold is ready. The mold istaped to a petri dish and 10:1 PDMS is poured over it and cured. Themold size ranges from ˜1 in² (for small samples) to 9 in² squares forthe well-plate embodiment. The sizing is arbitrary and easily scalable.

Once the stamp 41 is cleaned or otherwise treated, the chosen adhesivemolecule(s) are then inked onto the stamp 41 as seen in operation 155.For example, a protein solution is pipetted onto the stamp 41, allowedto wet the entire surface, and covered with a plastic sheet cut to sizeto help the solution spread over the full surface and prevent drying.This adsorption reaction happens for 30-60 minutes depending on theprotein used.

Once the adhesive molecule(s) have adsorbed to stamp 41, the stamp 41 isdried with pressurized air. Once dry the stamp 41 is immediately used tostamp the dextran-coated silicon wafer 43 as seen in operation 160.Dextran is purchased (Sigma-Aldrich) as a powder typically at the 70kDa-100 kDa sizes. A 20% mass-by-vol solution is prepared in deionizedwater in a test tube. The tube is mixed continuously (taped to a vortexmixer for example) for up to 30 minutes or until the dextran is fullydissolved. Silicon wafers are treated with plasma for 30 seconds toimprove their hydrophilicity and the dextran solution is spin-coatedonto the wafer to form a uniform sub-micron height coating. Afterspinning the dextran-coated wafers are baked at 150° C. for at leastfive (5) minutes to dry.

The stamping process includes rolling the stamp 41 with a cylindricalobject along several directions to aid in transferring the adhesive andfluorescent molecules from the PDMS stamp 41 to the dextran-coatedsilicon wafer 43. After rolling, a set of glass slides can be used as aweight and applied on the stamp 41 and kept in place for severalminutes. After transfer of the adhesive molecule(s) is complete, theweights can be removed and the stamp 41 is carefully removed from thedextran-coated wafer 43. Still referring to FIG. 9, as seen in operation165 uncured ultra-soft PDMS is spun onto the dextran coated wafer.

The ultra-soft PDMS mixtures are prepared in 50 mL tubes (or othertapered, closed container for best mixing). Crosslinker is first weighedout, then the appropriate amount of base is added. Working ratios aregenerally 50:1 to 70:1 though this range may expand to, for example,20:1 to 70:1 so the amount of crosslinker is very low, requiring one touse a closed container with preferably a tapered bottom that can beinverted and/or vortexed to promote optimal mixing. Afterinverting/vortexing for ˜3 minutes, the PDMS mixture is placed in vacuumto remove air bubbles (approximately 1 hr). Within about two (2) hoursof mixing the PDMS, the PDMS is spin-coated on the dextran-coated wafersthat have been stamped with protein or other molecule to form thepattern 16. The target height is about 10-15 microns. This height rangeis optimal for the well-plate format in terms of bonding thewell-containing plate 40 with the layer 12.

After the soft-PDMS mix is spin-coated onto the stamped dextran-coatedwafers, the wafers are left on a flat surface at room temp overnight toallow even distribution of the polymer (equal height), then placed intoan oven (60-80° C.) and cured until ready. The crosslinker used topolymerize the PDMS also acts to crosslink the transferred adhesivemolecule(s) and thus covalently incorporates the molecule(s) into thePDMS surface of the PDMS-dextran interface. Finally, as seen inoperation 170, the dextran is sacrificed (i.e., dissolved away) in water(or other solvent with high dextran solubility) which releases thethin-film PDMS layer 12 that incorporates thereon the molecular patterns16. A razor blade may be used to cut away a small amount of theperiphery of the PDMS layer 12 from the wafer to expose the dextran tosolution. After the periphery of the PDMS layer 12 is cut away a glassbacking layer that forms the optically transparent substrate 14 is thencontacted with the PDMS layer 12 and light pressure applied to ensuregood contact between the glass and the PDMS layer 12. After the glassbacking is added, the sample is submerged in PBS solution. An optionalshaker may be used to hasten the dissolving of the dextran layer. Thedextran layer quickly dissolves and the glass-backed PDMS layer 12floats freely in solution. The PDMS layer 12 and glass substrate whichforms the optically transparent substrate 14 is then carefully removedand inverted to be glass side down until it is ready to be sterilizedwith a strong base. The process of the thin-film PDMS layer 12 beingtransferred and mounted to the optically transparent substrate 14 isillustrated in operation 125. The process proceeds as previouslydescribed with the addition of a pluronic solution and seeding of cells11.

The pattern transfer and incorporation method described in the contextof FIG. 9 is preferred and has been used to pattern a number ofmolecules onto the layer 12. These include, Fibronectin, Fibrinogen,Bovine Serum Albumin, Ovalbumin, Streptavidin, Collagen Type I, CollagenType IV, Vitronectin, and IgG. The adsorption/lift-off method describedin FIG. 9 has been used with Fibronectin, Fibrinogen, and IgG. This isdue in part to the inability to treat soft PDMS with plasma as thiscreates a thin oxide layer than changes the mechanical properties of thesample. Other advantages of this pattern transfer approach include thecovalent coupling of protein into the polymer matrix which preventscell-induced breakage of patterns during high force generation.Dextran-coated wafers stamped with the adhesive and fluorescentmolecules are robust and storable. These non-hydrogel patternedsubstrates can be stored without hydration or refrigeration (both priorto and after PDMS has been spun on, but before the dextran release step)for at least several months improving robustness and allowing a lesscostly supply chain.

Cells 11 in a suspension, which can consist of cell lines brought intosuspension by trypsinization or cell scraping, or cells naturally foundin suspension in body fluids (e.g., blood, pleural fluid, urine,cerebral spinal fluid, etc.) are applied to the treated substrate andincubated for a period of time (between about 15 minutes to about 6hours) to adhere to the adhesive micro-patterns 16 and begin to applyforce or otherwise change the dimensions of the fluorescent pattern 16.The cells 11 may be monitored in a live state or they may be fixed with4% paraformaldehyde and imaged.

Experiments with Human Cells

The well-based embodiment described herein has been used to measuredpattern contraction and force generation when screening primary humanairway smooth muscle cells exposed to acetylcholine, enodthelin-1, andhistamine, and shown relaxation of smooth muscle contraction with ML-7and albuterol exposure. A 96 well plate implementation was used to forthe high throughput screening of these compounds that modulate and oraffect contractility of airway smooth muscle (ASM) cells. FIG. 11Aillustrates the contraction of ASM cells that were exposed toacetylcholine, enodthelin-1, and histamine. ML-7 was also added aftercontraction and was shown to relax smooth muscle contraction. This isseen by the reduction in deflection amount that was measured afteraddition of myosin light chain kinase inhibitor ML-7 (+ML-7). Thenegative control is also illustrated (−ML-7). FIG. 11B illustrates thecontraction of ASM cells in response histamine as well as the resultingrelaxation in response to the addition of albuterol (+ Albuterol). Theseresults illustrate that the system can be used in the high throughputscreening of candidate drugs for the treatment of inflammatory diseasestates that lead to smooth muscle contraction such as asthma.Multiparameter force phenotyping can be used on ASMs to identify linksbetween contractility and biomolecular content and organization, andpotentially identify molecular signatures of contractile sub-populationswith the most therapeutic importance for asthma and drug screening. Akey mechanism to treat asthma is through modulation of the contractionand proliferation of airway smooth muscle cells. This platform enablesthe high-throughput phenotypic screening method that targetscontractility. Compounds can be identified that amelioratebronchoconstriction and act through orthogonal signaling pathways to thecurrent successful β2-adrenergic receptor agonists that are part of thestandard of care.

FIGS. 12A and 12B illustrate how the platform and method can be used togenerate dose-response data for force phenotyping. In this experiment,HeLa cells were loaded into the well-based embodiment. Wells were loadedwith different concentrations of the myosin II inhibitor blebbistatin.Deflection measurements were made at the different concentration levels.In addition, relaxation of the HeLa cells was also measured. As seen inFIG. 12A, an increased concentration of blebbistatin resulted in lessdeflection. A dose-response curve is also illustrated in FIG. 12Bshowing the percentage of relaxation as function of blebbistatinconcentration.

Compatibility with Other Biological Studies

Broadly speaking, the system has applications in: (1) force phenotypingof cells (i.e., identifying cell sub-populations by unique adhesive andcontractile phenotypes rather than immunofluorescence); (2) drugdiscovery relating to cell contractility, and (3) diagnostics of immunedysfunction or immune state. The implementation described above, forexample, is useful for measuring inherent cell forces exerted throughfocal adhesions and provides a high-throughput method of phenotypingadherent mammalian cells based on the magnitudes of these forces.Specifically, this is a measure of how intrinsically contractile a givencell type is (i.e., its contractility). This assay is especially usefulin tracking differentiation of stem cells and other progenitor cells,and may be helpful in identifying specific cell types within mixedpopulations. Other implementations of this system provide ways ofassaying various other biological behaviors and responses to stimuli asnoted below.

Ligand-Mediated Forces:

The system can analyze the response of a given cell type to differentsurface-bound ligands. Since the system allows for any protein or othermolecule to be patterned on an ultra-soft layer 12, a cell type can bescreened against any number of ligands and the force-response can bemeasured in a well-defined manner. This may be used to createtissue-like structures or otherwise provide guidelines for tissueengineering.

Contractility Changes in Disease:

The system can be used to effectively quantify the differences incontractility between cells harvested from healthy tissue or organs andthose harvested from damaged or recovering tissues (scar tissue) ororgans. For example, contractility of myofibroblast or cardiomyocytestaken from a heart after a myocardial infarction could be compared totheir healthy counterparts. Drugs could then be screened that mayrestore healthy levels of contractility. Similarly, intestinal smoothmuscle cells, uterine smooth muscle cells and mesangial cells fromdisease models which have abnormal contractility can be compared tohealthy cells and drugs screened to restore healthy function (e.g.,force level of contraction).

Immune Function:

The system can be used to assay the immune state of a given organism bymeasuring various immunological functions of leukocytes, e.g. phagocyticability and contractile force of various phagocytes, and functions oflymphocytes, e.g. binding and contractile ability of T cells. This assaycan be used to diagnose immune function in patients and/or organismsquickly and in a functional way.

Phagocyte Function:

By patterning known opsonins such as antibodies, complement proteins andother circulating proteins as well as apoptotic bodies, the differentphagocytes comprising the immune system, e.g., monocytes, neutrophils,macrophages, among others can be tested for phagocytosing ability(quantifiable through analysis of pattern deformation and/or bymeasuring the amount of opsonins remaining after the phagocytes adhereand apply force to envelop the pattern). The forces applied are expectedto vary as a function of leukocyte type, activation, and opsonin(yielding patterns that are disease specific, e.g., for applications inmonitoring bacterial or viral infection, transplant rejection, orautoimmune conditions). Forces are expected to also scale with level ofimmune activity (e.g., immunosuppression vs. hyperactivity).

Lymphocyte Function:

The system can be used for comparing the applied forces and adherence oflymphocytes, such as T cells, taken from healthy patients to those takenfrom patients with autoimmune disease, e.g., chronic inflammatorydisease. T cells taken from patients with systemic lupus erythematosushave been known to display stronger actin polymerization, as well asincreased adhesion, both of which are behaviors that are measurable withthis system.

Allergen Assay:

The system can be used to pattern a variety of common allergens andscreen an individual's basophils and mast cells for an allergic responsein the form of increased binding, and therefore pattern deformation,which would be expected if the cells were decorated with large numbersof allergen-specific IgE, which is characteristic of an allergy.

Strain Due to Formation of Bio-Structures:

The system can be used to investigate and quantify the strain induced bythe formation of various bio-structures such as multicellular bacterialbiofilms as a function of structure size and duration of existence.Drugs could be screened that interfere with contractility of the biofilmor tissue-like structure (e.g., granuloma) which may be therapeuticallyuseful to disrupt the biofilm or disaggregate the granuloma.Additionally, spores produced by certain bacteria have been shown toexert differential strain on substrates that depends on environmentalconditions. See, e.g., Chen et al., O. Bacillus spores as buildingblocks for stimuli-responsive materials and nanogenerators. Nat Nano 9,137-141 (2014). This system can be used to make simple and statisticallysignificant comparisons of these responses against many environmentalstimuli and assist in the development of spore-based stimuli-responsivematerials.

Contractile Force Dependence on Cell Polarity:

The system can be used to study the contractile forces cells apply to asubstrate if constrained to highly asymmetric patterns, causingpolarization, which could aide in investigating biological processesinvolving cell polarity such as differentiation, proliferation, andmigration, all of which are critical for organism development andmaintenance. Since dysregulation of cell polarity is implicated indevelopmental disorders as well as cancer, drugs can be screened thatmay help control cell polarizability.

Effects of Electrical Stimulation on Contractility:

It has been shown that certain cell types align and/or elongate in adirection perpendicular to the direction of an applied electric field.The system may be used to now study cell contractile responses to suchexternal stimuli. These studies would be especially useful forquantifying the contractile forces of excitable cells (e.g., neuralcells, cardiomyocytes, smooth muscle cells) as well as the rates atwhich they are applied and related characteristics (e.g., periodicity).

Role of Genes in Contractility:

The system can be used in coordination with gene-silencing tools such asRNAi and CRISPR gene-editing technology to help identify which genes aremost responsible for cell contractility and elucidate the pathwaysthrough which multiple genes work together to control contractility.Since cancer progression may rely on increased cell contractility,particularly for its role in migration and remodeling of theextracellular matrix (ECM), the system may be used to identify possibledrug targets (e.g., proteins encoded by genes found to be implicated incontractility of malignant cells).

Unlike traction force microscopy and elastomeric micropost methods,which allow cells to adhere in uncontrolled, random and, therefore,irreproducible morphologies that make comparisons between experimentsdifficult, the system and method described herein precisely constrainscells to patterns designed by the user. Since the orientation, extent ofspreading, and polarization of adhered cells may dictate theirmechanical responses to stimuli, they are important variables that mustbe controlled for, and this is achieved with high reproducibility withthe patterning methods described herein. The invention utilizes a novelapproach for taking measurements of cell contractility by imaging andmeasuring the dimensions of the fluorescent patterns occupied by thecells rather than the cells themselves, which is the standard approach.Additionally, the new method we show of using a sacrificial dextranlayer and molding the proteins within the elastomeric matrix leads tohigher adhesive strength of the pattern to the elastomeric substratecompared to adsorption-based patterning, avoiding issues of cellspulling the pattern off the substrate when they contract. Furthermore,due to the well-defined target measurements, e.g., dimensions ofpatterns in pixels, custom-written automated software is used to parsethrough large volumes of data and measure the patterns with highaccuracy (tested against manual pixel length measurements), leading to ahigh-throughput platform compared to other platforms that must image athigher magnification or before and after a treatment at each individuallocation.

The platform technology disclosed herein will have several potentialmarkets. For example the approach can provide solutions to immunediagnostic problems (e.g., quickly diagnosing lupus, which isnotoriously difficult to detect with current commercial diagnosticsolutions), provide a drug screening platform for pharmaceuticaltesting, or serve as a research tool to characterize celldifferentiation. Generally, cell contractility is important to severalphysiological processes (e.g., cardiac function, immune cell function,smooth muscle function in various organs) and its dysregulation isimplicated in a variety of diseases. Current methods of quantifying cellcontractility have limitations in objectivity of measurements,through-put, and normalization of test conditions (e.g., cell morphologyand extent of cell-cell contact). Through the use of fluorescentadhesive molecules arranged in precise micro-patterns on the surfaces ofhighly flexible substrates, the invention provides a simple tool formaking well-defined quantitative measurements of cell contractilitywhile maintaining strict control over environmental conditions and cellorientation, spacing and spreading. As such, it presents a solution forscreening agents that affect cell contractility and could be useddiscover drugs and genes that restore healthy levels of contractility.

While embodiments of the present invention have been shown anddescribed, various modifications may be made without departing from thescope of the present invention. The invention, therefore, should not belimited, except to the following claims, and their equivalents.

What is claimed is:
 1. A system for assaying forces applied by cellscomprising: an optically transparent substrate comprising a softmaterial having a Young's modulus within the range of about 3 kPa toabout 100 kPa, the optically transparent substrate having a plurality ofwells formed thereon, the plurality of wells containing cells and one ormore different drugs or compounds; an array of geometrically shapedmolecular patterns disposed on a surface of the optically transparentsubstrate in the wells, the geometrically shaped molecular patternscomprising fluorophore-conjugated patterns adherent to cells; at leastone light source configured to excite the fluorophore-conjugatedpatterns; an imaging device configured to capture fluorescent lightemitted from the geometrically shaped molecular patterns; and acomputing device configured to receiving images from the imaging device,the computing device further configured to measure a dimensional changeof the geometrically shaped molecular patterns having cells disposedthereon.
 2. The system of claim 1, wherein the dimensional change ismeasured in real time.
 3. The system of claim 1, wherein the dimensionalchange is measured over an elapsed period of time.
 4. The system ofclaim 1, wherein the optically transparent substrate comprisespolydimethylsiloxane (PDMS).
 5. The system of claim 1, wherein thesystem comprises first and second light sources emitting light atdifferent wavelengths.
 6. The system of claim 1, wherein thegeometrically shaped patterns comprise a mixture or combination of twoor more proteins.
 7. The system of claim 1, wherein the array ofgeometrically shaped molecular patterns comprise symmetrical patterns.8. The system of claim 1, wherein the array of geometrically shapedmolecular patterns comprise asymmetrical patterns.
 9. A method forassaying forces applied by cells comprising: providing a test platecomprising: an optically transparent substrate comprising a softmaterial having a Young's modulus within the range of about 3 kPa toabout 100 kPa; an array of geometrically shaped molecular patternsdisposed on a surface of the optically transparent substrate, thegeometrically shaped molecular patterns comprisingfluorophore-conjugated patterns adherent to cells; a second substratecontaining a plurality of apertures therein, the second substratesecured to the surface of the optically transparent substrate to form aplurality of wells wherein each well contains one or more geometricallyshaped molecular patterns therein; providing at least one light sourceconfigured to excite the geometrically shaped molecular patterns;providing an imaging device configured to capture fluorescent lightemitted from the geometrically shaped molecular patterns; providing acomputing device configured to receiving images from the imaging device,the computing device further configured to measure a dimensional changeof the geometrically shaped molecular patterns having cells disposedthereon; loading the plurality of wells with cells so that at least someof the cells adhere to the geometrically shaped molecular patterns;loading some of the wells with a different compound or drug;illuminating the plurality of wells with the at least one light source;imaging the geometrically shaped molecular patterns from the pluralityof wells; and measuring a dimensional change of the geometrically shapedmolecular patterns within the wells with the computing device.
 10. Themethod of claim 9, further comprising identifying those cells having adimensional change within a certain range.
 11. The method of claim 9,wherein the dimensional change comprises contraction.
 12. The method ofclaim 9, wherein the dimensional change comprises relaxation.
 13. Themethod of claim 9, further comprising loading a fixation agent into theplurality of wells prior to measuring the dimensional change.
 14. Themethod of claim 9, wherein the dimensional change is measured in realtime.
 15. A method of identifying the force phenotype of cellscomprising: providing an optically transparent substrate comprising asoft material having a Young's modulus within the range of about 3 kPato about 100 kPa, the optically transparent substrate having an array ofgeometrically shaped molecular patterns disposed on a surface thereof,the geometrically shaped molecular patterns comprisingfluorophore-conjugated patterns adherent to cells; loading cells on theoptically transparent substrate, wherein at least some of thegeometrically shaped molecular patterns have one or more cells adheredthereto; illuminating the geometrically shaped molecular patterns; andcapturing images of fluorescent light emitted from the geometricallyshaped molecular patterns with an imaging device; measuring adimensional change of the geometrically shaped molecular patterns with acomputing device; and categorizing the cells based at least in part onthe measured dimensional change.
 16. The method of claim 15, wherein thecomputing device identifies those geometrically shaped molecularpatterns having a single cell disposed thereon.
 17. The method of claim15, wherein the cells are exposed to a compound or drug prior tomeasuring a dimensional change.